Hydraulic mounts are standard pieces of equipment for isolating engine vibration relative to a vehicle frame. They typically include a reinforced, hollow rubber body that is closed by a resilient diaphragm so as to form a cavity. The cavity is separated into a pumping chamber and a diaphragm chamber by a plate. The pumping chamber is formed between the partition plate and the body. The diaphragm chamber is formed between the plate and the diaphragm. The chambers are in fluid communication through an orifice in the plate. Restricted fluid flow through the orifice caused by displacement inputs to the pumping chamber results in viscous damping.
Most modern hydraulic mounts include a decoupler to reduce damping and stiffness of the mount for small displacement inputs. The decoupler is a flexible barrier between the pumping chamber and the diaphragm chamber. Small vibrational inputs primarily deflect the decoupler such that the decoupler movements alone accommodate small volume changes in the two chambers. In this manner, fluid flow between the chambers is substantially avoided at certain small vibrational amplitudes such that hydraulic damping does not occur.
In some hydraulic mounts, the decoupler is trapped between two perforated metal plates. For larger inputs, the decoupler “bottoms out” against the plates, forcing fluid to flow through the orifice and increasing damping and stiffness. The mount displacement at which the decoupler bottoms out is sometimes referred to as the “initiation of damping.”
Fluid in the holes of the perforated metal plates, together with the compliance of the mount's rubber body, form a resonant system. At frequencies above resonance, very little fluid will flow through the holes to the decoupler causing the decoupler to “choke off” and become ineffective. Accordingly, one objective in the design of a hydraulic mount is to make the frequency at which “choke off” occurs as high as possible. Ordinarily, this is accomplished by making the flow area of the holes, and therefore the area of the decoupler, as high as possible.
The disadvantage of increasing the area of the decoupler, however, is that for a given mount displacement input, a large area decoupler will undergo a physically smaller linear displacement than a small area decoupler. Therefore, to achieve the same “initiation of damping” a large area decoupler would need to be trapped between plates with more tightly controlled freeplay dimensions, which increases manufacturing costs.
The performance of hydraulic mounts can be improved through the use of magneto-rheological (“MR”) fluids. By subjecting these fluids to a magnetic field their flow characteristics can be altered to actively control fluid damping. Hydraulic mounts using MR fluid (“MR mounts”) typically use a fluid that is 2.4 times more dense than the hydraulic fluid used in standard hydraulic mounts. Consequently, to achieve the same “choke off” frequency as a standard hydraulic mount an MR mount would need a decoupler that is roughly 2.4 times larger in area. To achieve the same “choke off” frequency the freeplay dimensions of the decoupler would also need to be controlled 2.4 times more tightly. Accordingly, in the “choke off” frequency vs. manufacturing tolerance tradeoff, an MR mount with its denser working fluid is always at a disadvantage when compared to a standard hydraulic mount.
The object of the present invention is to provide a new MR mount that overcomes the these disadvantages by utilizing a lower density fluid as the working fluid for the decoupler.